1. Field of the Invention
This invention relates generally to voice communication over narrow band digital channels and more specifically to the passing of signalling tones in a transparent manner, i.e. without the need to detect them.
The novelty of the invention lies in the use of two signal encoders, one for speech and the other for tones, in a dynamic manner for a given signal frame. There is no need for handling signalling tones separately from speech signals.
2. Background
In narrowband digital voice communications, speech signals are compressed and encoded for transmission by means of speech coding. Speech coding, however, performs poorly on nonspeech signals such as DTMF tones or supervisory tones. When passing of such tones is required, it is typically handled separately from speech coding process by detecting the tones at the transmit side. The detector output, such as a DTMF digit or the presence of a certain tone, is then sent to the receiver via a signal or control channel to implement regeneration of corresponding tones at the receive side. This process is illustrated in FIG. 1.
This approach, however, assumes that the characteristics of tone detectors at the receive end are previously known since the signalling information is extracted at the transmit side. Another potential problem with this approach is the delay associated with tone detection and regeneration. For example, if a tone detector requires 100 milliseconds for detection, then the tone detector output will not be available until after 100 milliseconds.